1. Field of the Invention
The present invention relates to a wiring substrate having a signal wiring suitable for transmitting high frequency digital signals, and a process for producing the same. Especially, the invention relates to a wiring substrate having a specific signal-wiring configuration for enabling high-speed transmission of high frequency digital signals in digital systems having components such as CPU and main memory, and a process for producing the same.
2. Prior Art
Conventional wiring substrates and processes for producing them will be reviewed hereinafter.
In digital systems, digital signals, in other words, pulse signals, are used. In terms of sine-wave frequency convolution, the pulse signals have higher harmonic components. For example, a pulse signal of 200 MHz has third order sine-wave harmonics and fifth order sine-wave harmonics, with relative amounts measured in energy being about 30% and 10%, respectively. Thus, at the pulse signal of 200 MHz, it is required to design a transmission line configuration taking account of the frequency component of 1 GHz sine wave.
FIG. 12 shows a wiring substrate for a digital system having integrated circuit chips, which was examined by the inventors of the present invention as a typical example of the transmission line design for high frequencies. A CPU 2 and a main memory 3 are mounted on a wiring substrate 1. The CPU 2 and the main memory 3 are connected by a plurality of signal wirings 4, the number of which is determined by the number of the bits of the signal, for transmitting the digital signals.
Recently, as the developments in the high-speed CPU 2 are being accelerated, those with the operation frequencies as high as 400 MHz to 733 MHz have been developed. However, the frequency range of the digital signals allowed on the current print wiring boards is 133 to 200 MHz. This does not permit the signal wiring 4 to accommodate the higher frequencies of the digital signals transmitted form the CPU 2, resulting in a typical inconvenience that the data is not stored in the main memory 3.
Thus, the example above will need a development of a transmission line suitable for a frequency range of 2 GHz to 3 GHz in terms of the sine-wave frequency convolution of the digital signals. This proves that the design for transmission lines of a print wiring board now has to deal with GHz frequency range in digital systems.
Furthermore, in comparison to a RF (Radio Frequency) system, a digital system has a number of signals, more than 64 bits for example, being transmitted in parallel. This creates more difficult issues for the high frequency digital systems than the RF systems.
According to the present invention, it is possible to provide a wiring substrate structure and a manufacturing method thereof for transmitting high frequency digital signals.
The understanding of the phenomena involved in the transmission of the high frequency signals through the wiring lines is a crucial ingredient of the present invention, and now will be described hereinafter, before the description of the realization of the invention.
The difficulties confronted in the design of the high frequency digital signals in the GHz range are categorized in three issues. The first is that it is difficult to keep the characteristic impedance constant among the large number of bit wirings running in parallel. As the characteristic impedance changes depending on the interference from the neighboring wirings, it is required to eliminate most of the neighboring interference (the interference from the neighboring wirings).
A few designs for the high frequency wiring structure have been proposed in Japanese Laid-Open Patent Publication No. 11-284126. The idea is to suppress the neighboring interference by making a signal wiring pair by pairing two signal wirings, and by making the distance between signal and ground shorter than the distance between neighboring signal wiring pairs, for increasing the coupling between the two signal wirings composing a signal wiring pair.
One important point which needs attention is that higher reproducibility of the configuration and the size of the signal wiring is needed to keep the characteristic impedance constant, creating a need for a production process with even higher accuracy. Specifically, for example, the distance between the neighboring signal wiring pairs and the distance between the two signal wirings composing a signal wiring pair should be controlled so that the two distances are kept constant respectively. The wiring structures disclosed in the above Japanese Laid-open Patent Publication, however, do not satisfy this need.
The second is that the electric current flows only near the surface of the signal wiring while high frequency signals being supplied to the signal wiring, resulting in the reduction of the effective cross section of the electric current, and thereby in an increase in the DC current resistance and a decrease in the signal amplitude. This is called skin effect.
The inventors of the present invention have found that the skin effect is one of the major contributors degrading the transmission characteristics of the conventional signal wirings. Thus, the skin effect will be reviewed hereinafter extensively.
FIGS. 13A, 13B and FIGS. 14A, 14B are conceptual views for describing the skin effect occurring in the signal wirings. When DC current 6 flows through signal line 5 as shown in FIG. 13A, the DC current 6 flows with a same density all across the cross-section of the signal line 5.
If a high frequency signal is supplied to the signal line 5, the current flows in one direction, stops and then flows in opposite direction, and this sequence is repeated with a high speed. It is known that lines of magnetic force are generated around an electric current. Thus, while a high frequency signal is supplied, a cycle of generation and annihilation of the line of magnetic force is added to the sequence of the current flow comprising a flow in one direction, a stop of flow, and a flow in an opposite direction. The line of magnetic force can be perceived as having inertia in a similar manner as the mass has in dynamics. In other words, energies are required for generating line of magnetic force once the same is annihilated.
When the signal line 5 is considered as an assembly of a number of thin lines, the lines of magnetic force generated around the electric current elements 7 flowing through each thin line have the same direction. Thus, the lines of magnetic force generated around neighboring electric current elements collide each other and create a high-energy state.
Thus, following the principal of the nature, the electric current density becomes higher only near the surface of the signal line 5, as shown in FIG. 13B, so that the energy of the electro-magnetic system is minimized.
This happens, as the energy state is lower near the surface of the signal line 5 due to the suppressed mutual interference among the neighboring lines of magnetic force. This is the cause of the skin effect. However, having the skin with high electric current density all around the surface of signal line 5 is achieved under an assumption that a ground 9 is placed near the signal line 5.
Under the assumption above, the electric current flows only near the surface all around the contour of the signal line. The skin depth xcex4s is obtained by the following relation.
xcex4s=2/xcfx89xcexcxcex3"sgr"xe2x80x83xe2x80x83(1) 
And the skin depth is in proportional to 2xcfx80/xcfx89.
In the equation above, xcfx89 denotes an angular frequency, xcexcxcex3 the magnetic permeability of the signal line conductor, and "sgr" the electric conductivity of the signal line conductor. Assuming a Cu (Copper) wiring and 1 GHz sine wave, the skin depth is 2.2 xcexcm. More than 60% of the electric current flows within this depth, and almost 100% of the electric current flows within the depth of 5 xcexcm.
Now, the case of the signal wiring pair made by paring two signal wirings, or stacked pair lines, will be investigated hereinafter, as the same configuration constitutes an integral part of the present invention. It is assumed that a signal line 10 and a reference line 11 run in parallel as shown in FIG. 14A.
In this configuration, the currents flow in opposite directions to each other through the signal line 10 and the reference line 11. As a result, the direction of the lines of magnetic force generated by the electric current flowing through the signal line 10 is opposite to the direction of the lines of magnetic force generated by the electric current flowing through the reference line 11, and thus the lines of magnetic force of the opposite directions function to sustain themselves.
For minimizing the energy of the electromagnetic system, the electric current density becomes higher near the surfaces of the overlapping faces of the signal line 10 and the reference line 11, as indicated by the shaded area in FIG. 14A. On the both sides of the signal line 10 and the reference line 11, the electric current density becomes higher near the portion of the surface close to the overlapping faces. The skin depth xcex4s of the surfaces of the overlapping faces of the signal line 10 and the reference line 11 can be obtained by the equation 1 as an approximation.
In general, a reference line denotes a ground line which is represented by the idea that a ground is stable, or without xe2x80x9cfluctuationxe2x80x9d, as the earth ground is supposed to be. However, in reality, the electric potential of the ground line has xe2x80x9cfluctuationxe2x80x9d, and in recent electromagnetics it is referred as reference line or reference plane.
The mechanism of the generation of the skin effect will be further examined hereinafter using an equivalent circuit. FIG. 14B is an equivalent circuit of the signal line shown in FIG. 14A. In general, a signal line longer than the wavelength of the signal is represented by a distributed constant circuit having self-inductances LS1 and LS2, a mutual inductance M, and a coupling capacitance C.
Dividing the signal line 10 and the reference line 11 into the thin lines as shown in FIG. 14A, the equivalent circuit can be applied to the relation between the neighboring thin lines. The same equation can be applied to the relation between the signal line 10 and the reference line 11. Further, the signal line 10 and the reference line 11 can be divided into the thin lines in any manner suitable.
When the directions of the electric currents are identical, the state thereof is referred as a common mode and the effective inductance thereof is denoted by Leffc. On the other hand, the state is referred as a differential mode and the inductance is denoted by Leffd when the directions of the electric currents are opposite. This provides the following relations.
Leffc=LS1+LS2+2M, Leffd=LS1+LS2xe2x88x922M 
Thus, depending on the directions of the electric currents, the common mode is applied to the relations between the thin lines inside the signal line 10 and reference line 11, and the differential mode is applied to the relation between the signal line 10 and the reference line 11. This makes the electric currents flow through the low inductance region, and the electric current density becomes high near the surfaces of the overlapping faces of the signal line 10 and the reference line 11 keeping only limited electric current flow inside the both lines as shown in FIG. 14A. In other words, the electric currents are distributed so that the energy of the electromagnetic system is minimized.
As a result, the lines of electric force E and the lines of magnetic force H are generated outside the signal line 10 and the reference line 11 as shown in the same figure. The electromagnetic field around a wiring is generally extended into the ambience due to the fringe effect. However, the interference between the neighboring lines can be mostly eliminated by making the coupling coefficient close to 1 and thus localizing the electromagnetic field.
Then, the stacked pair lines will be compared to so-called micro strip lines for further understanding the characteristics of the skin effect. FIG. 15 is a conceptual view for describing the skin effect of the micro strip line. The figure has a signal line 12 and a reference plane 13. A virtual line appears as a imaginary part 14 of the signal line 12 with the reference plane 13 working as a symmetry plane.
In this configuration, the lines of electric force E are generated so that the signal line 12 and the imaginary part 14 are connected by the lines. The boundary condition is such that the lines of electric force E should be perpendicular to the reference plane 13. Thus, the lines of electric force E are extended along the reference plane 13 as shown in the same figure. Naturally, the electric current flows in an area wider than the width W of the signal line 12 on the surface of the reference plane 13. This is an advantage of a signal wiring pair under the effect of the skin effect since a signal wiring pair has less neighboring effect than a micro strip line.
In summary of the above discussion on the skin effect of a signal wiring, the electric currents flow near the surface all around contour of the signal line when a ground is placed in the ambience all around the contour of the signal line. However, in the micro strip lines and the stacked pair lines, the electric currents flow only near the surface facing a ground.
If the surfaces of the overlapping faces of the signal wiring pair are coarsened for obtaining an anchoring effect in expectation of a stronger adhesion between the metal wirings and the wiring substrates, the electric currents flow along the irregular contour of the rough surface, resulting in a longer electric current path and thus a lager DC current resistance loss. This further compounds the adverse influence of the skin effect on the transmission characteristics of the wiring substrates.
The third is that without reducing the dielectric loss, tan xcex4, of the insulating material surrounding the wirings the electric current leak between the wirings becomes large and dissipates as heat, resulting in a decrease in the signal energy. In the present invention, it is inevitable to use some form of insulating material between a couple of the wiring substrates for adhering the same. The amount of the electric current leak is represented by a leaking conductance G of the transmission lines.
The conductance above is obtained by the following relation.
G=kxcfx89C0tanxcex4
In the equation above, k denotes the insulator shielding rate of the transmission lines, xcfx89 the angular frequency, and C0 the capacitance between a signal and a ground in vacuum. As seen from the equation, increasing the angular frequency by one order of magnitude will require reducing tan xcex4 of the insulating material by one order of magnitude.
For reducing tan xcex4 of the insulating material, the polarized molecular structures in the insulating material should be eliminated as much as possible. In organic materials, this is to remove polarized groups from primary chains and sub chains.
On the other hand, metal wirings are adhered to organic materials through the polarized groups. Thus, there is an issue of the tradeoff between reducing the dielectric loss and obtaining high adhesion strength between organic materials and metal wirings. In general, the solution is to use the anchoring effect of organic materials to the irregular surface of the metal wirings, which are coarsened prior to the adhesion.
In the previous section, three issues are identified as difficulties or inconveniences in providing a wiring lines used with high frequency digital signals. The first issue is to eliminate the neighboring interference between signal wirings for lowering the characteristic impedance and keeping the same constant. The second issue is to minimize the increase in the electric resistance due to the skin effect occurring in signal wiring systems for preventing the attenuation of high frequency signals, and to enable the transmission of high frequency digital signals in GHz range. The third issue is to reduce the dielectric loss of insulating materials surrounding wirings for achieving the high efficiency transmission of high frequency digital signals.
Typical examples of the present invention will be summarized hereinafter.
The wiring substrate based on the 1st feature of the present invention includes:
one or a plurality of first signal wirings formed on a primary plane of a first substrate;
one or a plurality of second signal wirings formed on a primary plane of a second substrate;
insulating material inserted between a primary plane of the first substrate and a primary plane of the second substrate,
and the first and the second signal wirings are placed in parallel facing to each other for creating a signal wiring pair.
The structure above is the basic configuration of the wiring substrates of the present invention and enables high-speed transmission of high frequency signals without attenuations.
In the wiring substrate based on the 2nd feature of the invention which further defines the 1st feature of the invention, the surface roughness of the surfaces of the overlapping faces of the first and the second signal wirings is smaller than the skin depth of the skin effect caused by supplying high frequency signals to the signal wiring pair.
In this configuration, the electric current density becomes higher, due to the skin effect, near the surfaces of the overlapping faces of the first and the second signal wirings, and the increase of the electric resistance caused by the skin effect is minimized since the roughness of the surfaces of the overlapping faces of the first and the second signal wirings is smaller than the skin depth due to the skin effect. Ideally, a mirror plane free from the surface roughness is desirable, but making the surface roughness smaller than the skin depth due to the skin effect can sufficiently suppress the increase in the electric resistance. Thus, the attenuation of the high frequency signals can be prevented and high-speed signal transmission can be achieved.
In the wiring substrate based on the 3rd feature of the invention which further defines the 1st and the 2nd features of the invention, where a plurality of sets of the signal wiring pairs are formed close to each other, the first and the second wirings are placed so that the following relation is obtained among the distance between the first and the second signal wirings, t, the line width of the first and the second wiring, a, and the distance between neighboring signal wiring pairs, b:
b/(a+t) greater than 2. 
In this configuration, the coupling coefficient becomes close to 1 and the neighboring interference between neighboring signal wiring pairs can be practically annihilated. Thus the same configuration can provide an ideal wiring without the loss of the electromagnetic energy.
In the wiring substrate based on the 4th feature of the invention which further defines the 3rd feature of the invention, the signal wiring pairs have a characteristic impedance lower than 15 xcexa9. The same configuration is desirable for transmitting high frequency digital signals in GHz range.
In the wiring substrate based on the 5th feature of the invention which further defines the 1st feature of the invention, the insulating material fills up the entire gap between the first and the second substrates. In this configuration, the insulating materials are used for joining the first and the second substrates. It is desirable to choose insulating materials with low dielectric constant, or low tan xcex4, for reducing the leak current due to the dielectric loss between the signal wirings.
In the wiring substrate based on the 6th feature of the invention which further defines the 1st feature of the invention, the insulating material is partially inserted into the space between the neighboring signal wiring pairs, and a gas is sealed into the space surrounded by the first and the second signal wirings, and the insulating material.
In the 5th feature of the invention, as the insulating material fills the entire space, the dielectric characteristics is generally degraded even when a low tan xcex4 material is used, compared to the case where a gas is inserted. Thus, in the wiring substrate of the 6th feature of the invention, a gas is sealed into the space between the overlapping faces, which is the area for the highest electromagnetic energy and, thus, dominates the dielectric loss characteristics.
In this configuration, as the space between the overlapping faces of the first and the second signal wirings is filled with a gas, only the fringe effect accounts for the dielectric loss. Thus, the dielectric loss thereof can be neglected even when the insulating material supporting the substrates has more or less high tan xcex4.
In the wiring substrate based on the 7th feature of the invention which further defines the 1st feature of the invention, the insulating material is placed partially in the space between the neighboring signal wiring pairs away from the edge of the signal wiring pairs, and a gas is sealed into the space surrounded by the first and the second signal wirings, and the insulating material. This configuration can eliminate the dielectric loss including the fringe effect occurring at the signal wiring pairs. Further, this configuration has a structure which makes it easier to control the distance between the surfaces of the signal wiring pair than the structure of the 6th feature of the invention. This advantage will be described in detail hereinafter.
In the wiring substrate based on the 8th feature of the invention which further defines the 1st feature of the invention, the insulating material is formed as a coating layer on the first and the second substrates having the first and the second signal wirings thereon, the coating layers are joined together on the first and the second signal wirings, and a gas is sealed into the space between signal wiring pairs.
In this configuration, it is easy to control the distance between the signal wirings of the signal wiring pairs as the thickness of the coating layer determines the same distance. It is desirable to form the coating layer using a material with a low tan xcex4.
In the wiring substrate based on the 9th feature of the invention which further defines the 6th, 7th, and 8th features of the invention, the gas is a non-polarized gas. The candidates for the non-polarized gas include helium, argon, methane, ethane, and the air from which at least the moisture is removed.
The wiring substrate based on the 10th feature of the invention includes:
a plurality of substrates laminated using insulating materials in between, so that first signal wirings and second signal wirings formed on the substrates are placed in parallel facing to each other for forming a plurality of layers of signal wiring pairs;
buried via formed through the substrates or formed through the substrates and supporting structures for the substrates in the layers for interconnecting the signal wiring pairs in the layers.
This configuration permits the lamination of the signal wiring pairs using the wiring substrate of the 1st feature of the invention as a basic structural unit, and enables high-density wiring substrates.
In the wiring substrate based on the 11th feature of the invention which further defines the 10th feature of the invention, pads for electric connection are formed on the substrate in the upper most layer among the plurality of laminated substrates, and the pads for electric connection are connected to the signal wiring pairs in the layers by buried via formed through the substrates or by buried via formed through the substrates and the insulating material in the layers. Then, electric components are mounted on the substrate in the upper most layer using the pads for electric connection.
This configuration enables the mounting of electronic components onto the wiring substrates having laminated signal wiring pairs, which is disclosed in the 10th feature of the invention.
In the wiring substrate based on the 12th feature of the invention which further defines the 10th and the 11th feature of the invention, the buried via is formed on via lands extended from the first signal wirings and the second signal wirings.
The wiring substrate based on the 13th feature of the invention which further defines the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th, and 12th features of the invention, further includes:
a first integrated circuit chip having a driver circuit for transmitting complementary signals to the first and the second signal wirings;
a second integrated circuit chip having a receiver circuit for receiving complementary signals transmitted through the first and the second signal wirings.
This configuration enables the high-speed transmission of the signals form the first integrated circuit chip to the second integrated circuit chip.
In the wiring substrate based on the 14th feature of the invention which further defines the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th, and 12th features of the invention, the insulating materials are epoxy resin, bis (maleide) triazine (BMT), polyimide, benzocyclobutane (BCB), or fluorinated resin. As the insulating materials above contain no, or at least a very few, polarized groups, the dielectric constant becomes significantly small. One of the examples of the fluorinated resin used is aromatic fluorence.
The manufacturing method of wiring substrates based on the 15th feature of the invention includes:
a process for forming one or a plurality of first signal wirings on a primary plane of a first substrate;
a process for forming one or a plurality of second signal wirings on a primary plane of a second substrate;
a process for coarsening the surface of the first and the second substrates except the surface of the first and the second signal wirings;
a process for forming insulating materials on the entire surface of the first and the second substrates;
a process for joining the first substrate and the second substrate using the insulating materials in between, and the first and the second signal wirings are placed in parallel facing to each other for forming signal wiring pairs.
This configuration permits the manufacturing of the wiring substrates based on the 1st and 5th features of the inventions. In this configuration, the first and the second substrates undergo a process for coarsening the surfaces thereof for improving the adhesion, however no treatment for coarsening is applied to the surfaces of the signal wirings. By keeping the smooth surfaces of the signal wirings, the increase of the electric resistance due to the skin effect is minimized.
In the manufacturing method of wiring substrates based on the 16th feature of the invention which further defines the 15th feature of the invention, among all the processes, at least, the process for joining the first substrate and the second substrate using the insulating materials in between is performed in a gas atmosphere composed of non-polarized molecules.
This configuration prevents the intake of the polarized molecules, such as water molecule, into the insulating material, and thus prohibits the increase in tan xcex4 of the insulating material.
In the manufacturing method of wiring substrates based on the 17th feature of the invention which further defines the 15th, and the 16th invention, the insulating materials are epoxy resin, bis (maleide) triazine (BMT), polyimide, benzocyclobutane (BCB), or fluorinated resin. As the insulating materials above contain no, or at least a very few, polarized groups, the dielectric constant becomes significantly small. One of the examples of the fluorinated resin used is aromatic fluorence.
In the manufacturing method of wiring substrates based on the 18th feature of the invention which further defines 15th, and 16th features of the invention, the insulating material is thermosetting resin or thermo-plastic resin.
The manufacturing method of wiring substrates based on the 19th feature of the invention includes:
a process for adhering a photosensitive dry film onto a primary plane of a first and a second substrates;
a process for creating a first and a second openings at the regions for a first and a second signal wiring to be formed on the first and the second substrates using patterning of the photosensitive dry film;
a process for forming the first and the second signal wirings in the first and the second openings using electroless plating;
a process for joining the first substrate and the second substrate using the photosensitive dry films filled between the first and the second signal wirings, and the first and the second signal wirings are placed in parallel facing to each other for forming signal wiring pairs.
This configuration enables the manufacturing of the wiring substrates based on the 6th feature of the invention.
In the manufacturing method of wiring substrates based on the 20th features of the invention which further defines 19th feature of the invention, among all the processes, at least, the process for joining the first substrate and the second substrate using the photosensitive dry films filled between the first and the second signal wirings is performed in a gas atmosphere composed of non-polarized molecules. This configuration prevents the intake of the polarized molecules, such as water molecule, into the photosensitive dry films, and thus prohibits the increase in tan xcex4 of the insulating material.
The manufacturing method of wiring substrates based on the 21st feature of the invention includes:
a process for forming one or a plurality of first signal wirings on a primary plane of a first substrate;
a process for forming one or a plurality of second signal wirings on a primary plane of a second substrate;
a process for coarsening the surface of the first and the second substrates except the surface of the first and the second signal wirings;
a process for forming columns made of insulating materials partially in the space between the neighboring signal wiring pairs on the surface of the first and the second substrates;
a process for joining the first substrate and the second substrate using the columns made of the insulating material in between,
and the first and the second signal wirings are placed in parallel facing to each other for forming signal wiring pairs.
This configuration enables the manufacturing of the wiring substrates of the 7th feature of the invention. Furthermore, it is possible to adjust the column height so that the distance between the signal wirings of the signal wiring pairs is kept constant.
In the manufacturing method of wiring substrates based on the 22nd feature of the invention which further defines 21st feature of the invention, among all the processes, at least, the process for joining the first substrate and the second substrate using the columns in between is performed in a gas atmosphere composed of non-polarized molecules. This configuration prevents the intake of the polarized molecules, such as water molecule, into the columns, and thus prohibits the increase in tan xcex4 of the insulating material.
The manufacturing method of wiring substrates based on the 23rd feature of the invention includes:
a process for forming one or a plurality of first signal wirings on a primary plane of a first substrate;
a process for forming one or a plurality of second signal wirings on a primary plane of a second substrate;
a process for coarsening the surface of the first and the second substrates except the surface of the first and the second signal wirings;
a process for forming coating layers by evaporating organic materials on the surface of the first and the second substrates having the first and the second signal wirings;
a process for joining the first substrate and the second substrate using the coating layer in between, and the first and the second signal wirings are placed in parallel facing to each other for forming signal wiring pairs.
This enables the manufacturing of the wiring substrates of the 8th feature of the invention. Furthermore, as the thickness of the coating layer determines the distance between the signal wirings of the signal wiring pairs, the same distance can be easily controlled. It is desirable to form the coating layer using materials with low tan xcex4.
In the manufacturing method of wiring substrates based on the 24th feature of the invention which further defines 23rd feature of the invention, among all the processes, at least, the process for joining the first substrate and the second substrate using the coating layer in between is performed in a gas atmosphere composed of non-polarized molecules. This configuration prevents the intake of the polarized molecules, such as water molecule, into the coating layer, and thus prohibits the increase in tan xcex4 of the insulating material.
In the manufacturing method of wiring substrates based on the 25th feature of the invention which further defines 16th, 20th, 22nd, and 24th features of the invention, the gas atmosphere composed of non-polarized molecules is helium, argon, methane, ethane, or the air with at least the moisture being removed thereof.